Many types of printing systems include one or more printheads that have arrays of marking elements that are controlled to make marks of particular sizes, colors, etc. in particular locations on the print media in order to print the desired image. In some types of printing systems, the array of marking elements extends across the width of the page, and the image can be printed one line at a time. However, the cost of a printhead that includes a page-width array of marking elements is too high for some types of printing applications, so a carriage printing architecture is used.
In a carriage printing system (whether for desktop printers, large area plotters, etc.) the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath. In a carriage printer, the marking element arrays are typically disposed along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction. The length of the marking element array determines the maximum swath height that can be used to print an image.
In single-pass printing, each marking element that is used for printing is responsible to print all pixel locations that are required in a corresponding raster line of the image swath. After printing the swath, the page is advanced by a distance corresponding to the length of the marking element array and the next swath is printed, again with each marking element being responsible to print all pixel locations that are required in the corresponding raster line of that image swath. Single pass printing has the advantage of fast print throughput, and is frequently used in draft printing modes. However, in practice, marking elements are nonuniform in a variety of ways. They can produce nonuniform dot sizes on the recording medium. They can be misdirected such that the dot location is displaced from its intended location. They can be defective such that no dot at all is produced. Such nonuniformities produce objectionable image quality defects in single-pass printing.
In multi-pass printing, responsibility for printing each raster line of the image is shared between a plurality of marking elements. In this way the nonuniform marking behavior of marking elements can be disguised in order to provide improved image quality. Multi-pass printing can also enable multi-tone printing in which multiple dots are printed in the same pixel locations, and can also provide time for improving the uniformity of ink-media interactions by controlling the pattern of dots that can be printed within one pass. Multi-pass printing is described in more detail in commonly assigned co-pending U.S. Patent Publication No. 2007/0201054 A1.
In order to ensure that each pixel location of the image can be printed during at least one of the m passes in a multi-pass print mode, a print mask is provided for each color plane of the image. The print mask is typically a two dimensional array of rows and columns of Boolean data. Each row of the print mask contains 1's and 0's for each corresponding marking element in the marking element array indicating which pixel locations are authorized for printing by that marking element during the printing of a swath of data. In other words, the print mask data is ANDed with the image data in order to indicate which pixel locations can be printed by each marking element in a given print swath. For single-tone m-pass printing (where each pixel location can receive one and only one dot during the printing of the m passes), the print mask is composed of m mask sections, where each mask section includes complementary mask data, such that each row of data in one mask section is complementary to corresponding rows in the other mask sections.
In normal multi-pass printing, after each swath of data is printed, the recording medium is advanced by a distance corresponding to the length of the marking element array divided by m. If there are a total of M marking elements in the array that prints the swath, every (M/m)th marking element shares responsibility for printing a given line of the image. Therefore, a set of marking elements separated by the total number of marking elements divided by m is sometimes called a set of complementary marking elements.
It is found that while normal multi-pass printing is effective in disguising print quality defects due to nonuniform marking elements, there can still be banding defects that are observable in the image, such as chromatic banding. Observability of such banding defects can be reduced, if the page is not always advanced by a distance corresponding to the length of the marking element array divided by m, but is rather irregularly advanced. Irregular page advance is disclosed, for example in U.S. Pat. Nos. 6,336,702 and 6,866,358.
The problem with irregularly advancing the page by different amounts is that a single-mask configuration having complementary mask sections no longer has the complementary rows of mask data lining up in successive print passes. As a result, some pixel locations cannot be printed. One way to solve this is to use different print masks for different print passes, but this complicates the printing and also results in excessive memory requirements.
What is needed is a method for compensating for irregular page advances of the recording medium during multi-pass printing.